Ceramic media are commonly used in a wide range of fluid handling procedures, including filtration, diffusion, recovery, transfer, mixing and foaming. Ceramic media are also employed as catalysts, or as carriers for catalysts. Ceramics are well known to possess several advantages as media for fluid handling over alternatives such as organic or metallic filter media. For example, ceramics generally possess superior resistance to deterioration from heat or chemical exposure, in comparison to other media.
Ceramic media are most often used in the form of an aggregation of ceramic particles, either loose or bound to one another. The ceramic particles can be formed as spheres, platelets or needles. The particles are routinely obtained by crushing and classifying (that is, sorting by size) a previously manufactured mass of a desired material. This method of manufacture is subject to some drawbacks, however. Crushing of a formed material can often degrade some of the desirable structural characteristics enjoyed by the material, such as its impact strength, mechanical strength, rigidity, porosity or aspect ratio. (With respect to materials such as platelets or needles, a lower aspect ratio indicates either a shorter or a thicker needle. Thinner needles yield filters with smaller pore sizes.) Moreover, with materials such as platelets or needles, the separately formed platelets or needles are often very delicate and interwoven when manufactured, and consequently fracture upon attempts to separate and classify them.
Even presuming that such particles can be successfully separated, aggregation of the particles may require sintering or the use of a bonding agent, in order to give rigidity, impact strength or mechanical strength. Alternatively, the particles may be aggregated by placing them in a metal container. However, such containers can be expensive, and may not be completely resistant to the gas or liquid being treated. Moreover, it can be difficult to achieve a good seal between the metal container and the particles; if a good seal is not obtained, the fluid may leak around and bypass the particles.
Ceramic media have also been used as supports for porous discriminating layers such as fluorocarbon polymers or sintered ceramic membranes. These supports have typically been made from previously fired spherical particles of alpha-alumina or cordierite. The particles are then lightly sintered to bond them together and give them mechanical strength. Unfortunately, the resulting supports may not possess all the strength that might be desired, particularly against impact, or against the pressure of the fluid flowing through them. The resulting supports have also not been very porous, usually only about 30 percent porous (or 70 percent of theoretical density, defined as 100 percent minus the volume percent porosity).
Many solutions to these problems have been suggested. Each entails its own drawbacks, however. For example, published Japanese Patent application JP 63-103877A (Nagasaki Ken, published May 9, 1988) discloses a process for preparing a porous ceramic compact useful for industrial filtering, for foam generating, as a bioreactor carrier, or for catalysis. The compact is described as having fine porous structure with a relatively high deflection strength. The compact consists of acicular mullite crystals formed from the compression molding and sintering of stoichiometric mullite (3 Al.sub.2 O.sub.3 .multidot.SiO.sub.2): The starting material includes additives so as to allow transfer of any unreacted or any excess silica into a glass phase, which is then eluted with an acid. Although not specifically stated in this reference, hydrofluoric acid has typically been employed for this purpose.
U.S. Pat. No. 3,993,449 (Howard Jacobson et et al., Nov. 23, 1976) discloses a process for preparing single crystal mullite fibrils useful as fillers, catalysts, or catalyst supports. The fibrils are made from aluminum sulfate, a silica source and an alkali metal salt (fluxing agent). The molar ratio of aluminum to silicon in the reactants is from 2.6:2 to 6:2, expressed as Al.sub.2 O.sub.3 /SiO.sub.2, with at least one alkali metal atom for each aluminum atom. The reference states that although a product that is predominantly "true" (3/2 or stoichiometric) mullite can be obtained from reactants throughout that range, it is preferred to maintain the ratio in the range 2.8 to 3.4:2, so as to avoid the quantity of alpha-alumina platelets which are obtained if there is a large excess of alumina in the reactant mixture.
While both of these disclosures suggest that a fibrous mullite body or support can be obtained that is relatively strong, the degree to which the whiskers forming the bodies bind to each other is not clear. Moreover, control of pore size in such bodies is not as great as could be desired, because the average pore sizes are typically quite small. The bodies are often not useful for applications requiring higher porosities, for example, from 50 up to 85 percent. The use of hydrogen fluoride to elute the glass phase is itself inconvenient because of the risks involved in handling hydrofluoric acid. Additionally, devices constructed from metal generally cannot be used in processes employing hydrofluoric acid as an elution agent.
These problems are especially acute in methods which entail the use of cross-flow structures, for example, cross-flow filtration or heat exchanging. Ceramic cross-flow filters or heat exchangers typically have very low flow rates through them, because of the small pore size and low porosity of their supports. Indeed, even when permeable membranes of very fine pore size are applied to ceramic supports, it is still the rate of flow through the ceramic supports which is the rate-determining factor for the desired filtration or heat exchange.
U.S. Pat. No. 4,894,160 (Fumio Abe et al., Jan. 16, 1990) attempts to address these problems by providing a honeycomb structure for fluid filtration which comprises a porous ceramic support having a multiplicity of parallel passageways formed through it by uniformly spaced porous partition walls, permitting pressurized fluid to flow through the support. A selective membrane is coated onto the surface of the passageways to separate one or more components from the fluid. The filtrate is carried through the partition walls to the exterior surface of the partition walls for collection. The porous partition walls are formed so as to permit the passage of filtrate at a flow quantity more than 20 times the flow quantity of filtrate passing through the selective membrane and partition walls. The selective membrane has an average pore size of about 10 to 10,000 angstroms (about 1 to 1,000 nanometers). The device is useful for microfiltration or ultrafiltration, or for gas separation on the basis of gas diffusion or capillary condensation.
In the preferred embodiment of the device of the reference, the honeycomb support element is contained within a cylindrical casing and held in place by a pair of support plates near the ends of the ceramic support element. The cylindrical casing includes a discharge port for collection of filtrate exiting the partition walls. The external surface of the partition walls between the support plates is coated with a glaze, to direct the flow of filtrate into the discharge port.
While useful for its intended purpose, the device shown by the reference would appear to be subject to several drawbacks during use. The joints between the honeycomb structure and the support plates, or between the support plates and the cylindrical housing, may be subject to leakage, especially during repeated thermal cycling. Moreover, the honeycomb support is disclosed as being constructed of alpha-alumina and kaolin, necessarily sharing the drawbacks of the prior ceramic supports. The ceramic support has a pore volume of about 0.07 to 0.25 cubic centimeters per gram, or a porosity of merely 7 to 25 percent. This low porosity places an upper limit on the pore size of the selective membrane disposed on the porous partition walls. The device is thus plainly subject to the prior drawbacks encountered by supports composed of conventional ceramics or made from a plurality of different components of different materials.
It is therefore an object of the present invention to provide a ceramic filter structure with a discriminating layer thereon, where the filtering medium has been grown in situ to form a network of interlocked needles or platelets which has high mechanical strength, high impact strength, heat resistance and good resistance to thermal cycling.
It is a further object of the present invention to provide a support for a ceramic or other filter membrane having high porosity, yet with superior bonding of the material making up the support for the filter.
It is yet another object of the present invention to provide a method for manufacturing such a filter including a ceramic support which does not entail the dangers associated with the prior use of hydrogen fluoride or other acid for eluting a glass phase from the support.
It is a further object of the present invention to provide a method of interlocking two or more pieces into a ceramic body having a uniform composition and structure, including throughout the locations at which the pieces are joined.
It is also an object of the present invention to eliminate the need for elements of disparate composition, and the need for any means to connect such elements, in a ceramic structure for filtration, heat exchange, or the like.